Beat detection generally refers to detection of cardiac depolarizations. Detection of cardiac depolarizations is an important, often essential, part of cardiac signal analysis for diagnosing irregular or abnormal rhythms of a patient's heart. A heart functions as an electromechanical pump which forces blood to circulate throughout the body via the body's circulatory system to provide for the body's metabolic needs. The mechanical pumping function is accomplished with contractions of myocardium (heart muscles), which includes excitable tissues constructed of cardiac myocytes (contractile heart muscle cells). When the heart is at rest, the myocardium maintains a resting electrical potential through its cell membranes. In the absence of electrical current within the myocardium, two electrodes placed in or about the myocardium in its resting state would record no electrical signal. As the myocardium is excited by a sequence of electric events, self-propagating action potentials result from a complex cascade of electric currents that flow across the cell membranes. Consequently, a depolarizing wave of action potential sweeping through the myocardium is recorded by the two electrodes while causing the myocardium to contract. Thus, a cardiac depolarization is recorded by a pair of electrodes as an indication of a heart contraction, referred to as a heart beat. A beat can be detected by detecting the depolarization wave.
A temporal pattern of heart beats is known as cardiac rhythm. Depending on the location of the recording electrodes, cardiac rhythm reflects depolarizations at specific cardiac regions such as the right atrium, the left atrium, the right ventricle, and the left ventricle. In a normal heart, the depolarizations and other related cardiac events as recorded at various cardiac regions are well coordinated and synchronized with certain delays. When the heart functions irregularly or abnormally, however, the depolarizations and other related cardiac events as recorded at various cardiac regions may be chaotic and unsynchronized, indicating irregular or other abnormal cardiac rhythms, known as cardiac arrhythmias. Cardiac arrhythmias result in a reduced pumping efficiency of the heart, and hence, diminished blood circulation. Examples of such arrhythmias include bradyarrhythmias, that is, hearts that beat too slowly or irregularly, and tachyarrhythmias, that is, hearts that beat too quickly.
A cardiac rhythm management system includes a cardiac rhythm management device used to treat cardiac arrhythmia by delivering electrical pulses to the patient's heart. Cardiac rhythm management devices include, among other things, pacemakers, also referred to as pacers. Pacemakers are often used to treat patients with bradyarrhythmias. Such pacemakers may coordinate atrial and ventricular contractions to improve the heart's pumping efficiency. Cardiac rhythm management devices also include devices providing cardiac resynchronization therapy (CRT), such as for patients with congestive heart failure (CHF). CHF patients have deteriorated heart muscles that display less contractility and cause poorly synchronized heart contraction patterns. By pacing multiple heart chambers or multiple sites within a single heart chamber, the CRT device restores a more synchronized contraction of the weakened heart muscle, thus increasing the heart's efficiency as a pump. Cardiac management devices also include defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Such defibrillators may also include cardioverters, which synchronize the delivery of such stimuli to portions of sensed intrinsic heart activity signals. Defibrillators are often used to treat patients with tachyarrhythmias. In addition to pacemakers, CRT devices, and defibrillators, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacemakers and defibrillators, drug delivery devices, and any other systems or devices for diagnosing or treating cardiac arrhythmias.
All modern cardiac rhythm management systems require detection of cardiac depolarizations to detect cardiac arrhythmias and to determine the nature of a detected cardiac arrhythmia and an appropriate therapy treating it. In one example, a fast, irregular pattern of ventricular depolarizations, known as R-waves, may indicate a ventricular fibrillation treatable by delivering a defibrillation pulse from a defibrillator. In another example, a heart having a normal atrial rhythm but poor atrioventricular synchrony can be resynchronized by pacing a ventricle following a predetermined delay after each atrial depolarization, known as a P-wave, is detected.
Therefore, a reliable beat detection, or detection of cardiac depolarizations, is essential to an accurate diagnosis of a cardiac arrhythmia and a successful and efficient administration of a therapy using a cardiac rhythm management system. Beat detection has been traditionally accomplished by using threshold criteria based on a first or second derivative of a sensed cardiac signal. A depolarization is detected whenever the amplitude of the cardiac signal exceeds such a threshold. Failure of detection occurs when, for example, noise is present, cardiac repolarizations (T-waves) are recorded as events having high amplitude or large slopes in the cardiac signal, and the slew-rate of the cardiac signal decreases.
For these and other reasons, the present inventors have recognized a need for ensuring a more reliable beat detection.